Seismic compression is defined as the accrual of contractive volumetric strains in soils during earthquake shaking and has been recognized as a major cause of seismically induced damage to geotechnical infrastructure. Seismic compression is challenging to predict for unsaturated soils, which are widely encountered in engineered geostructures like embankments or retaining walls involving compacted backfills and in near-surface natural soil layers. This study involves development of a new experimental setup and methodology to characterize the volumetric contraction of unsaturated sands during cyclic shearing under different drainage conditions. Specifically, a new cyclic simple shear device with suction-saturation control and embedded pore air and water pressure sensors was developed to perform cyclic shearing tests on unsaturated sand specimens with different initial suctions in drained, partially drained, or undrained conditions. This device can measure the changes in volumetric strain, degree of saturation, and pore air and water pressures, which can help in characterizing the evolution in effective stress and hydromechanical coupling effects during cyclic shearing. The device also permits evaluation of the shear modulus and damping ratio for cyclic shear strain amplitudes ranging from 0.1 to 10%.

Sand specimens under nearly-saturated, dry and unsaturated conditions in the funicular regime of the SWRC were assessed using the new cyclic simple shear device under different drainage conditions. During drained cyclic shearing (constant suction), volumetric strains caused a shift in the soil-water retention curves to higher degrees of saturation. This led to a greater effective stress for the same matric suction and enhanced the resistance of unsaturated specimens to seismic compression. The stabilized volumetric strain after drained cyclic shearing followed a log-linear relationship with matric suction. During undrained cyclic shearing, seismic compression volumetric strains followed a nonlinear relation with degree of saturation. In addition to the initial volume of pore air, the evolution in mean effective stress resulting from hydromechanical coupling played a significant role on the magnitude of seismic compression during undrained cyclic shearing. The pore air and water pressures were found to both increase but at different rates, leading to a decrease in matric suction and effective stress. In both drained and undrained cyclic shearing, the shear modulus was found to increase due to seismic compression. The data from this study confirm the usefulness of the new device and were used to form the basis for a new constitutive soil model for cyclic shearing of unsaturated sand.